A conventional rotation detection sensor for detecting a rotation stopped condition of a rotatable body using a light beam comprises a single optical sensor and a rotatable disk (referred to hereunder simply as a disk) with an outer peripheral portion thereof having approximately equal width transparent portions which pass a light beam and opaque portions which do not pass a light beam. The optical sensor has a light source which generates an AC light beam by means of an AC signal from a signal generator, and a light receiving element for receiving the light beam from the light source, positioned facing each other on opposite sides of the outer peripheral portion of the disk, and generates from an output circuit, an output signal corresponding to a light received output from the light receiving element.
With such a construction, when the disk is rotating, the transparent portions and the opaque portions alternately intersect the light beam. Since the widths of the transparent portions and the opaque portions are approximately the same, then the output from the light receiving element of the optical sensor has an output form with a high level condition and a low level condition appearing alternately at approximately even interval following to the passing of the transparent portions and the opaque portions with rotation of the disk. Since the light source is driven by an AC signal from the signal generator, then in the case where the signal frequency of the signal generator is substantially higher than the rotational speed of the disk, the output signal from the optical sensor in the high level interval where the light beam is received, becomes an AC signal which is switched at the signal frequency of the signal generator.
When the disk stops rotating, then either a transparent portion or an opaque portion is positioned in the optical path of the light beam. Consequently, the output condition of the optical sensor is fixed at either a high level condition or a low level condition.
In this arrangement, when the disk stops rotating, an indicator lamp is illuminated, for example, for advising personnel that the vicinity of the disk is "not dangerous". In order to illuminate the indicator lamp with an output signal from the optical sensor, when the disk has stopped, a light beam is passed so that the output signal from the optical sensor becomes a high level condition, the indicator lamp can be illuminated directly by the output energy from the optical sensor. However, the disk will not always stop at a position where the light beam is passed. In the case where the disk stops at a position where the light beam is shut off, then the indicator lamp cannot be illuminated directly by the output signal from the optical sensor.
Therefore, it has been proposed that a rotation detection sensor has two indicator lamps, one having the output signal from the optical sensor input directly thereto, and the other having the output signal from the optical sensor input via a NOT circuit thereto, so that when the disk stops, one or the other of the two indicator lamps is continuously illuminated. In this case, when the disk is rotating the two indicator lamps flash alternately.
It has also been proposed that a rotation detection sensor has a circuit configuration with a single indicator lamp, which is devised such that the indicator lamp is continuously illuminated when the disk stops.
More specifically, the circuit configuration is such that a light received output from an optical sensor is rectified by a rectifying circuit, and a rectified output therefrom is then input to an on-delay circuit via a NOT circuit. An indicator lamp is then illuminated by an output from the on-delay circuit. The rectifying circuit in this case is a device for rectifying the envelope of the AC output signal from the optical sensor which is based on the passing and shut off of the light beam due to rotation of the disk. It has no rectifying function with respect to the AC signal having the frequency of the signal generator. That is to say, the envelope of the AC output signal from the optical sensor due to the passing of the transparent and opaque portions of the rotatable disk is treated as the AC signal, and this AC signal is rectified by the rectifying circuit.
In this way, whether the disk stops in a condition such that the light beam is received by the light receiving element via a transparent portion, or stops in a condition where the light beam is shut off by an opaque portion, once stopped, the output condition from the optical sensor is fixed and hence does not become alternating. Consequently, since if the disk is stopped, the rectified output from the rectifying circuit is always at a zero level, and the output signal from the NOT circuit is thus always a high level, then the indicator lamp can be illuminated after a predetermined time lapse by the output from the on-delay circuit.
However, with either of the abovementioned circuit configurations, the function of the NOT operation carried out by the NOT circuit is indispensable for illumination of the indicator lamp when the disk is stopped. Therefore the following problems arise.
In the case where the transparent portions of the disk become soiled, there is the possibility that even when a transparent portion intersects the light beam, light is not received by the light receiving element. In this case, the light received output level of the optical sensor will remain at a low level condition even if the disk is rotating. The indicator lamp to which the signal is input via the NOT circuit will therefore be continuously illuminated, indicating that the disk is stopped (rotation stopped) even though the disk may still be rotating. This situation can arise not only in the abovementioned case where the transparent portions become soiled. For example, the same can also occur in the case where the light source malfunctions so that light is not produced, or the light receiving element malfunctions so that light cannot be converted to an electrical signal, or the light source or light receiving element attachment becomes disconnected (in the case where the light receiving element is provided independent of the output circuit).
The present invention takes into consideration the above situations with the object of providing a rotation detection sensor which can advise of a rotatable body stopped condition by for example illumination of an indicator lamp, without using a NOT circuit to carry out a NOT operation on the output signal from the optical sensor.